Polyurethane flexible foam and method for its production

ABSTRACT

The present invention relates to a polyurethane flexible foam having a resonant frequency of not higher than 3.5 Hz and a method for producing such a polyurethane flexible foam. 
     The polyurethane flexible foam of the present invention is obtained by using at least one member selected from a specific polyoxyalkylene polyol and a polymer-dispersed polyol containing said polyoxyalkylene polyol as the matrix and a polyisocyanate compound as the main reactive materials, and it is a polyurethane flexible foam having a resonant frequency of not higher than 3.5 Hz. 
     Such a polyurethane flexible foam is obtained by using a polyoxyalkylene polyol with both the hydroxyl value (X mgKOH/g) and the total unsaturation degree (Y meq/g) being particularly low, or a polymer-dispersed polyol containing such a polyoxyalkylene polyol as the matrix. Namely, it is obtained by using as the above polyoxyalkylene polyol, a polyoxyalkylene polyol having a hydroxyl value (X mg/KOH/g) of from 5 to 38 and a total unsaturation degree (Y meq/g) of not more than 0.07, and both being in the following relation: 
     
         Y≦0.9/(X-10) 
    
     As such a polyoxyalkylene polyol, a polyoxyalkylene polyol having an oxypropylene group content of at least 70% by weight and an oxyethylene group content of at least 5% by weight, is preferred. 
     Further, it is preferred to use a small amount of a crosslinking agent together with the above polyoxyalkylene polyol. 
     The polyurethane flexible foam of the present invention has a vibration characteristic suitable for seat cushions for automobiles. Accordingly, a comfortable ride in an automobile can be improved by using the polyurethane flexible foam of the present invention for a seat cushion of the automobile.

DESCRIPTION

1. TECHNICAL FIELD

The present invention relates to a polyurethane flexible foam and amethod for its production. Particularly, it relates to a polyurethaneflexible foam having excellent physical properties as a seat cushion forautomobiles, and a method for its production.

2. BACKGROUND TECHNIQUE

In recent years, various new studies have been made to improve theproperties of polyurethane flexible foam. For example, reflecting atrend for grading up of e.g. automobiles, improvement of the vibrationcharacteristics of seat cushions, is desired. The relation between thevibration of the car body and the human being is not uniform. However,it is suggested that it is effective for the improvement of thecomfortable ride to take a particularly large attenuation of thefrequency region (e.g. from 4 to 8 Hz or from 6 to 20 Hz) to which humanbeing is particularly sensitive. Accordingly, it is considered possibleto substantially improve the comfortable ride if the seat cushion ismade of a polyurethane flexible foam having a resonant frequency lowerthan this frequency region. However, heretofore, no polyurethaneflexible foam has been known which has a resonant frequency of less than4 Hz.

SUMMARY OF THE INVENTION

The present invention provides a polyurethane flexible foam having aresonant frequency of less than 4 Hz. Further, the present inventionprovides a method for producing a polyurethane flexible foam, wherebysuch a polyurethane flexible foam can be produced. The polyurethaneflexible foam of the present invention can be obtained by using apolyoxyalkylene polyol having a particularly low hydroxyl value and alow total unsaturation degree. Namely, it can be obtained by using asthe polyoxyalkylene polyol a high molecular weight polyol which is apolyoxyalkylene polyol containing oxyalkylene groups of at least 3carbon atoms as the main component, particularly a polyoxyalkylenepolyol having at least 70% by weight of oxypropylene groups derived from1,2-propyleneoxide and wherein the hydroxyl value (X mgKOH/g) and thetotal unsaturation degree (Y meq/g) are in the relation of the followingformula (1), and Y ≦0.07 and X is from 5 to 38:

    Y≦0.9/(X-10)                                        (10)

The average number of hydroxyl groups per molecule of thispolyoxyalkylene polyol is preferably from 2 to 8, more preferably from 2to 6. Further, a polymer-dispersed polyol having this polyoxyalkylenepolyol as the matrix and a mixture of such a polymer-dispersed polyoland this polyoxyalkylene polyol, are also preferred polyols.

Further, it is also preferred to use a polyfunctional compound having arelatively low molecular weight which is called a crosslinking agent,together with the above polyoxyalkylene polyol. It is essential to use apolyisocyanate compound as a starting material for the polyurethaneflexible foam. Further, a catalyst, a foaming agent and a foamstabilizer are usually essential assisting agents. Thus, thepolyurethane flexible foam of the present invention is produced by usingthese materials as the starting materials.

For the polyurethane flexible foam of the present invention, it isessential that its resonant frequency is less than 4 Hz, particularlynot higher than 3.5 Hz. In addition, the impact resiliency of the coreis preferably at least 70%, and further, the wet heat permanent strainis preferably not higher than 10%.

Now, each starting material component in the present invention will bedescribed.

Polyoxyalkylene polyol

The polyoxyalkylene polyol used as a starting material for polyurethane,is usually produced by ring-opening addition polymerization of analkylene oxide such as propylene oxide to an initiator such as apolyhydric alcohol by means of an alkali catalyst such as an alkalimetal hydroxide. In such a process, a monool having an unsaturated groupis formed as a byproduct, and the production rate of this unsaturatedmonool increases with an increase of the molecular weight of the polyol(with a decrease of the hydroxyl value). The presence of thisunsaturated monool is not in such a serious amount in the case of apolyoxyalkylene polyol having a hydroxyl value of about 56 which iswidely used as a starting material for polyurethane flexible foam.However, in a polyoxyalkylene polyol having a low hydroxyl value whichis used as a starting material for e.g. polyurethane elastomer, thepresence of this unsaturated monool is likely to create a problem. Forexample, with a polyoxyalkylene polyol having a hydroxyl value of about34, the total unsaturation degree usually becomes at least 0.1 meq/g.Further, it has been practically impossible to prepare a polyoxyalkylenepolyol having a still lower hydroxyl value by means of an alkalicatalyst, since its total unsaturation degree becomes remarkably high.

Further, even if an flexible foam is produced by using a polyoxyalkylenepolyol having a high total unsaturation degree, there will be problemssuch as a decrease in the hardness, a decrease in the impact resiliency,a deterioration in the compression permanent strain, a deterioration inthe curability at the time of molding the foam, etc.

The polyoxyalkylene polyol used in the present invention has a lowunsaturation degree as compared with a starting material commonlyemployed for polyurethane flexible foam, whereby the conventionalproblems, particularly the problems observed in the case of employing ahigh molecular weight polyoxyalkylene polyol having a hydroxyl value ofnot higher than 38 (a decrease in the hardness, a decrease in the impactresiliency, a deterioration in the compression permanent strain and adeterioration in the curability at the time of molding the foam) can becontrolled, and it is excellent in the vibration attenuation properties.

Such a polyoxyalkylene polyol can usually be obtained by using acatalyst other than an alkali catalyst, for example, by using zincdiethyl, iron chloride, metal porphyrin or a double metal cyanidecomplex, as the catalyst. A particularly excellent polyoxyalkylenepolyol is obtainable by the use of a double metal cyanide complex.Methods for producing such a polyoxyalkylene polyol are disclosed, forexample, in the following documents:

U.S. Pat. No. 3,829,505, U.S. Pat. No. 3,941,849, U.S. Pat. No.4,355,188,

U.S. Pat. No. 3,427,334, U.S. Pat. No. 3,427,335, U.S. Pat. No.4,472,560,

U.S. Pat. No. 4,477,589, EP 283,148

In the present invention, as the polyol, at least one member of theabove-mentioned polyoxyalkylene polyols having a low unsaturation degreeand low hydroxyl value, may be employed. Further, in addition to thepolyoxyalkylene polyol, a polyol having from 2 to 8, particularly from 2to 4 hydroxyl groups, such as a polyester-type polyol, a hydroxylgroup-containing polydiene-type polymer or a polycarbonate-type polyol,may optionally incorporated, as the case requires. It is particularlypreferred to use at least one polyoxyalkylene polyol or apolymer-dispersed polyol containing such a polyoxyalkylene polyol as thematrix, or to use it as the main component in combination with a smallamount (usually not more than 30% by weight) of a polyester-type or ahydroxyl group-containing polydiene-type polymer.

As the polyoxyalkylene polyol in the present invention, polyhydricalcohols, saccharides, alkanolamines, polyhydric phenols,polyoxyalkylene polyols obtained by adding alkylene oxides thereto andhaving molecular weights lower than the desired products, orpolyoxyalkylene polyols obtained by adding at least one type ofmonoepoxide to other initiators, may be used. As the monoepoxide, it ispreferred to use 1,2-propylene oxide, 1,2-butylene oxide, 2,3-butyleneoxide, styrene oxide, at least one of other alkylene oxides having atleast 3 carbon atoms, and a combination of at least one of them withethylene oxide. Particularly preferred is at least one of 1,2-propyleneoxide, 1,2-butylene oxide and 2,3-butylene oxide, or a combination of atleast one of them with ethylene oxide. Most preferred is apolyoxyalkylene polyol containing at least 70% by weight, particularlyat least 80% by weight, of oxypropylene groups derived from1,2-propylene oxide.

As the above polyoxyalkylene polyol, a polyoxyalkylene polyol having ahigh primary hydroxyl value is preferred. Such a polyoxyalkylene polyolhas oxyethylene groups or polyoxyethylene block chains at its molecularterminals. The proportion of such oxyethylene groups present at theterminal portions is preferably at least 2% by weight, more preferablyfrom 5 to 30% by weight, most preferably from 5 to 20% by weight.

The polyoxyalkylene polyol in the present invention may be a mixture oftwo or more different types. However, it is preferred that it does notsubstantially contain a polyoxyalkylene polyol with a hydroxyl valueexceeding 38. In such a case, the hydroxyl value, the number of hydroxylgroups and the total unsaturation degree are represented by averagevalues. In the present invention, the polyoxyalkylene polyol has theupper limit of Y being 0.07 when X is not higher than about 22.9. Morepreferably, the upper limit of Y is 0.04 (when X is from about 32.5 to38, the formula (1) will apply). Further, the hydroxyl value is morepreferably from 5 to 28.

Polymer-dispersed polyol

As the polyol of the present invention, a polymer-dispersed polyol canbe employed. The polyol as the matrix for the polymer-dispersed polyolis required to be the above-mentioned polyoxyalkylene polyol. Thepolymer-dispersed polyol is a dispersion having fine polymer particlesdispersed stably in this matrix. As the polymer, an additionpolymerization polymer or a condensation polymerization polymer may bementioned. A polymer-dispersed polyol wherein the matrix is aconventional polyol, is known and is widely used a polyol forpolyurethane flexible foam. The polymer-dispersed polyol in the presentinvention can be produced by a conventional method using the abovepolyoxyalkylene polyol as the matrix. Further, a relatively small amountof a conventional polymer-dispersed polyol may be added to the abovepolyoxyalkylene polyol. In such a case, a polyol constituting the matrixof the conventional polymer-dispersed polyol will be added in a smallamount to the above polyoxyalkylene polyol. Even in such a case, therelation of the hydroxyl value and the total unsaturation degree of theabove polyoxyalkylene polyol is required to be satisfied as an average.Thus, the polyoxyalkylene polyol constituting the matrix for thepolymer-dispersed polyol in the present invention, is required tosatisfy the above requirements (e.g. the relation of the hydroxyl valueand the total unsaturation degree).

The fine polymer particles in the polymer-dispersed polyol are made ofan addition polymerization type polymer such as a homopolymer orcopolymer of acrylonitrile, styrene, methacrylate, an alkylmethacrylate, an alkyl acrylate or other vinyl monomer, or acondensation polymerization type polymer such as a polyester, polyurea,polyurethane or a melamine resin. By the presence of such fine polymerparticles, the hydroxyl value of the entire polymer-dispersed polyolwill usually be lower than the hydroxyl value of the polyol as thematrix. Accordingly, the hydroxyl value of the entire polymer-dispersedpolyol having the above polyoxyalkylene polyol as the matrix, ispreferably from 5 to 38, more preferably from 5 to 28.

The content of the fine polymer particles in the polymer-dispersedpolyol or in a mixture of such a polymer-dispersed polyol and the abovepolyoxyalkylene polyol, is usually not higher than 60% by weight,preferably not higher than 40% by weight. The amount of the fine polymerparticles is not required to be large. On the other hand, even if theamount is excessive, there is no particular disadvantage other than theeconomical disadvantage. In most cases, they are sufficiently effectivein an amount of not higher than 20% by weight. The presence of finepolymer particles in the polyoxyalkylene polyol is not essential.However, the presence is effective for the improvement of the hardness,the air permeability and other physical properties of the foam.Accordingly, the fine polymer particles are preferably present in anamount of at least 0.1% by weight, more preferably at least 1% byweight, most preferably at least 2% by weight.

Crosslinking agent

In the present invention, it is possible to react only the abovepolyoxyalkylene polyol having a low hydroxyl value (i.e. a highmolecular weight) (provided that water is excluded) with the isocyanatecompound. However, a low molecular weight polyfunctional compoundreactive with an isocyanate group (which is called a crosslinking agentin the present invention) can be used together with the high molecularweight polyoxyalkylene polyol. This polyfunctional compound ispreferably a compound having at least two isocyanate-reactive groupssuch as hydroxyl groups, primary amino groups or secondary amino groupsand having a molecular weight of not higher than 600, particularly nothigher than 300, per isocyanate-reactive group. Such a crosslinkingagent includes a compound which is usually called a crosslinking agentor a chain extender in the polyurethane technical field. Such a compoundincludes, for example, polyhydric alcohols, alkanolamines, polyamines,and low molecular weight polyoxyalkylene polyol type polyols obtained byadding small amounts of alkylene oxides to polyhydric alcohols,akanolamines, saccharides, polyamines, monoamines or polyhydric phenols.Further, low molecular weight polyester type polyols or polyamines mayalso be employed. Preferably, polyhydric alcohols such as ethyleneglycol, propylene glycol, 1,4-butanediol and glycerol, alkanolaminessuch as diethanolamine and triethanolamine, polyoxyalkylene type polyolshaving a hydroxyl value of at least 200, and polyamines such ast-butyltolylenediamine, diethyltolylenediamine and chlorodiaminobenzene,are employed. Particularly preferred are polyoxyalkylene type polyolshaving a hydroxyl value of at least 200 and from 3 to 8 hydroxyl groups.Such a polyfunctional compound is used preferably in an amount of nothigher than about 10 parts by weight, particularly not higher than 5parts by weight, per 100 parts by weight of the high molecular weightpolyol. There is no particular restriction as to the lower limit of theamount. However, in its use, adequate effectiveness is obtainable withan amount of about 0.2 part by weight.

Polyisocyanate compound

The polyisocyanate compound may be an aromatic, alicyclic or aliphaticpolyisocyanate having at least two isocyanate groups, a mixture of atleast two such polyisocyanates, as well as modified polyisocyanatesobtained by modifying them. Specifically, there may be mentioned, forexample, polyisocyanates such as tolylene diisocyanate (TDI),diphenylmethane diisocyanate (MDI), polymethylenepolyphenyl isocyanate(so called crude MDI), xylylene diisocyanate (XDI), isophoronediisocyanate (IPDI) and hexamethylene diisocyanate (HMDI), and theirprepolymer type modified products, nulate modified products, ureamodified products and carbodiimide modified products. Preferably, atleast one aromatic polyisocyanate selected from TDI, MDI, crude MDI andtheir modified products, is employed. More preferably, a mixturecomprising at least one of MDI, crude MDI and their modified productsand TDI or its modified products, is employed. The amount of TDI ispreferably from 50 to 100% by weight.

The isocyanate group content in the polyisocyanate compound in thepresent invention is preferably at least 15% by weight, particularlypreferably at least 20% by weight.

Other starting material components

For the reaction of the polyol and the polyisocyanate compound, it isusually required to employ a catalyst. As the catalyst, a metal compoundcatalyst such as an organotin compound for accelerating the reaction ofthe active hydrogen-containing group with the isocyanate group, or atertiary amine catalyst such as triethylenediamine, may be used.Further, a polymerization catalyst for reacting isocyanate groups to oneanother, such as a metal salt of a carboxylic acid, may be used as thecase requires. Further, in many cases, a foam stabilizer for forminggood foam, is used. As the foam stabilizer, a foam stabilizer ofsilicone type or a foam stabilizer of fluorine-containing compound typemay, for example, be mentioned. Other optional additives include, forexample, a filler such as silica, alumina or calcium carbonate, anantioxidant, an ultraviolet absorber, a stabilizer such as aphotostabilizer, a colorant and a flame retardant. As mentioned above,in the present invention, as the foaming agent, a water-type foamingagent, a halogenated hydrocarbon such as R-11 (trichlorofluoromethane),R-12 (difluorodichloromethane), R-123(1,1-dichloro-2,2,2-trifluoroethane), R-12B₂ (difluorodibromomethane),R-141b (1,1-dichloro-1-fluoroethane) or methylene chloride, butane,hexane, air or nitrogen, may be employed. The water-type foaming agentmay be water itself or a compound capable of generating water at thetime of the reaction, such as a hydrate. A low boiling point halogenatedhydrocarbon may be used as a foaming agent in combination with thewater-type foaming agent.

In a case where a water type foaming agent is employed as the foamingagent, a part of the above polyisocyanate compound reacts with water togenerate carbon dioxide gas. Accordingly, the amount of thepolyisocyanate compound used is calculated on the basis of the sumobtained by adding the water-type foaming agent to the total of the highmolecular weight polyol and the low molecular weight polyfunctionalcompound, and it is preferred to use the polyisocyanate compound in anamount of from 0.8 to 1.3 equivalent relative to 1 equivalent of thesum. 100 Times of the equivalent of this polyisocyanate compound isusually called (isocyanate) index. Accordingly, the index of thepolyisocyanate compound is preferably from 80 to 130.

EXAMPLES

The following polyoxyalkylene polyols were used as starting materialsfor polyurethane flexible foams described hereinafter. Each polyol wasprepared by adding propylene oxide to an initiator of polyoxypropylenepolyol having a molecular weight of from 400 to 600 by means of a zinchexacyanocobaltate complex as a catalyst, then deactivating thecatalyst, and adding ethylene oxide by means of an alkali catalyst,followed by purification to remove the catalyst component. In Table 1,the names of the obtained polyoxyalkylene polyols, and the numbers ofhydroxyl groups (N), the hydroxyl values (X), the oxyethylene contents(EO) and the total unsaturation degrees (Y) of the polyoxyalkylenepolyols, are shown.

The polyols used for the purpose of comparison (polyols J, K, and L) arepolyoxyalkylene polyols prepared by reacting propylene oxide to aninitiator (glycerol) by means of an alkali catalyst, and then reactingethylene oxide thereto, followed by purification.

                  TABLE 1                                                         ______________________________________                                        Name    N     EO (wt %)  X (mg KOH/g)                                                                             Y (meq/g)                                 ______________________________________                                        Polyol A                                                                              3     15         24         0.020                                     Polyol B                                                                              3     10         10         0.025                                     Polyol C                                                                              4     10         10         0.025                                     Polyol D                                                                              3     12         17         0.023                                     Polyol E                                                                              3     12         11         0.026                                     Polyol F                                                                              3     10         14         0.025                                     Polyol G                                                                              3     15         28         0.020                                     Polyol H                                                                              3     15         32         0.018                                     Polyol I                                                                              4     17         28         0.020                                     Polyol J                                                                              3     15         28         0.10                                      Polyol K                                                                              3     15         32         0.15                                      Polyol L                                                                              3     15         34         0.102                                     ______________________________________                                    

Polymer-dispersed polyols:

Polymer-dispersed polyols containing the above polyoxyalkylene polyolsas matrix and containing 20% by weight of fine particles ofpolyacrylonitrile, were used. The names of such polymer-dispersedpolyols are represented by adding p to the names of the respectivepolyols (example: polyol Ap).

Other materials used for the preparation of polyurethane flexible foamsare as follows.

Crosslinking agent:

A: A sorbitol-propylene oxide-ethylene oxide adduct. Hydroxyl value: 450(molecular weight of about 125 per hydroxyl group)

B: Diethanolamine Catalysts:

A: Triethylenediamine solution ("Dabco 33LV" tradename)

B: N ethylmorpholine

C: Stannous octoate

D: Tertiary amine catalyst ("Kaolizer No. 1" tradename, sold by KaoCorporation)

Foaming agent:

A: Water

B: 1,1-Dichloro-2,2,2-trifluoroethane (R-123)

C: 1,1-Dichloro-1-fluoroethane (R-141b)

Foam stabilizer:

A: Silicone type foam stabilizer ("L 5750S" tradename, sold by NipponUnika K.K.)

B: Silicone type foam stabilizer ("L-5309" tradename, sold by NipponUnika K.K.)

C: Silicone type foam stabilizer ("SF-2962" tradename, sold by ToraySilicone K.K.)

D: Silicone type foam stabilizer ("SRX-274C" tradename, sold by ToraySilicone K.K.)

Polyisocyanate compounds:

A: TDI-80

B: Mixture of TDI-80 and crude MDI ("MR-200" tradename, sold by NipponPolyurethane K.K.) in a weight ratio of 80/20.

C: Modified MD2 for polyurethane flexible foam (NCO content: 27%)

EXAMPLES 1 to 17

Using the starting materials as identified in Table 2, polyurethaneflexible foams were prepared.

The polyisocyanate compound was added to a mixture of all the startingmaterials except for the polyisocyanate compound, followed by stirring.The mixture was immediately injected into a mold of 350 mm x 350 mm and100 mm in height heated to 60° C. or 40° C. (No. 1 and No. 17), and themold was closed and left at room temperature for 5 minutes. Then, themolded polyurethane flexible foam was taken out from the mold. Then, thefollowing physical properties of the foam were measured. The types ofthe starting materials, the amounts (represented by parts by weight,provided that the amount of the polyisocyanate compound used wasrepresented by the isocyanate index (100 times of the equivalentratio)), and the physical properties of the foam are shown in Table 2.The physical properties were measured by the following methods.

Appearance: Visually evaluated

Air flow: ASTM D1564

25% ILD: JIS K6401

Impact resilience: JIS K6401

Elongation: JIS K6401

Wet heat permanent strain: JIS K6401

Resonant frequency: JASO B407-82

6 Hz transmittance: JASO B407-82

                                      TABLE 2                                     __________________________________________________________________________                       1    2   3   4    5    6    7     8    9                   __________________________________________________________________________    Composition                                                                          Polyol (amount)                                                                           A (100)                                                                            B (100)                                                                           C (100)                                                                           A (70)                                                                             D (70)                                                                             E (70)                                                                             C (70)                                                                              A (70)                                                                             E (70)              of starting                     Ap (30)                                                                            Dp (30)                                                                            Ep (30)                                                                            Cp (30)                                                                             Ap                                                                                 Ep (30)             material                                                                             Crosslinking agent                                                                             A (3.0)                                                                           B (1.0)                                                                           A (3.0)                                                                            A (3.0)                                                                            A (3.0)                                                                            A (3.0)                                                                             A (3.0)                                                                            A (3.0)                    (amount)                                                                      Catalyst (amount)                                                                         A (0.3)                                                                            A (0.6)                                                                           A (0.6)                                                                           A (0.7)                                                                            A (0.7)                                                                            A (0.7)                                                                            A (0.7)                                                                             A (0.7)                                                                            A (0.7)                                B (0.3)                                                                            D (0.5)                                                                           D (0.5)                                                              C (0.2)                                                           Foaming agent                                                                             A (3.5)                                                                            A (3.5)                                                                           A (3.7)                                                                           A (3.0)                                                                            A (3.0)                                                                            A (3.0)                                                                            A (3.0)                                                                             A (3.0)                                                                            A (2.8)                    (amount)                                      B (3.0)                                                                            B (5.0)                    Foam stabilizer                                                                           A (2.0)                                                                            C (1.5)                                                                           C (1.0)                                                                           D (1.0)                                                                            D (1.0)                                                                            D (1.0)                                                                            D (1.0)                                                                             D (1.0)                                                                            D (1.0)                    (amount)                                                                      Polyisocyanate                                                                            A [100]                                                                            A [105]                                                                           C [105]                                                                           B [105]                                                                            B [105]                                                                            B [105]                                                                            B [105]                                                                             B [105]                                                                            B [105]                    [Index]                                                                Physical                                                                             Appearance  Good Good                                                                              Good                                                                              Good Good Good Good  Good Good                properties                                                                           Core density (kg/m.sup.3)                                                                 41.0 38.0                                                                              49.9                                                                              47.6 47.9 48.2 47.2  47.3 47.0                       Air flow (cfm)                                                                            --   --  --  2.3  2.9  3.3  1.8   3.6  2.8                        25% ILD (kg/314 cm.sup.2)                                                                 18.7 15.8                                                                              23.6                                                                              22.7 21.8 20.0 21.7  23.3 20.0                       Impact resiliency                                                                         74   80  79  80   84   87   80    78   85                         (core) (%)                                                                    Elongation (%)                                                                            205  165 125 149  155  165  139   130  153                        Wet heat compression                                                                      9.9  9.7 3.5 5.0  4.7  4.3  7.0   8.4  5.3                        permanent strain (%)                                                          Resonant frequency                                                                        3.4  3.2 3.1 3.0  2.8  2.7  3.1   3.0  3.0                        (Hz)                                                                          6 Hz transmittance                                                                        0.50 0.40                                                                              0.38                                                                              0.35 0.30 0.20 0.40  0.35 0.37                __________________________________________________________________________                              10   11  12   13   14  15   16   15                 __________________________________________________________________________           Composition                                                                          Polyol (amount)                                                                           E (70)                                                                             F (70)                                                                            G (70)                                                                             H (70)                                                                             I (70)                                                                            K (70)                                                                             K                                                                                  L (100)                   of starting        Ep (30)                                                                            Fp (30)                                                                           Gp (30)                                                                            Hp (30)                                                                            Ip (30)                                                                           Kp (30)                                                                            Kp (30)                        material                                                                             Crosslinking agent                                                                        A (3.0)                                                                            A (3.0)                                                                           A (3.0)                                                                            A (3.0)                                                                            A (3.0)                                                                           A (3.0)                                                                            A (3.0)                               (amount)                                                                      Catalyst (amount)                                                                         A (0.7)                                                                            A (0.7)                                                                           A (0.6)                                                                            A (0.6)                                                                            A (0.6)                                                                           A (0.7)                                                                            A                                                                                  A (0.3)                                                                       B (0.3)                                                                       C (0.2)                          Foaming agent                                                                             A (3.0)                                                                            A (3.0)                                                                           A (3.3)                                                                            A (3.3)                                                                            A (3.3)                                                                           A (3.3)                                                                            A                                                                                  A (4.0)                          (amount)    C (3.0)                                                                            B (3.0)           B (3.0)                                                                            B (3.0)                               Foam stabilizer                                                                           D (1.0)                                                                            D (1.0)                                                                           D (1.0)                                                                            D (1.0)                                                                            D (1.0)                                                                           D (1.0)                                                                            D                                                                                  A (2.0)                          (amount)                                                                      Polyisocyanate                                                                            B [105]                                                                            A [105]                                                                           B [105]                                                                            B [105]                                                                            A [105]                                                                           B [105]                                                                            A                                                                                  A [100]                          [Index]                                                                Physical                                                                             Appearance  Good Good                                                                              Good Good Good                                                                              Cure Cure Good                      properties                                defect                                                                             defect                                                                   Foam Foam                                                                     partially                                                                          partially                                                                destroyed                                                                          destroyed                             Core density (kg/m.sup.3)                                                                 51.3 46.9                                                                              48.3 47.9 48.2                                                                              47.1 48.0 36.0                             Air flow (cfm)                                                                            2.4  3.5 3.0  2.4  3.3 3.5  3.3  --                               25% ILD (kg/314 cm.sup.2)                                                                 23.3 22.0                                                                              23.5 26.1 26.6                                                                              21.0 21.5 15.7                             Impact resiliency                                                                         75   83  73   71   74  62   63   48                               (core) (%)                                                                    Elongation (%)                                                                            163  150 141  132  140 90   110  145                              Wet heat compression                                                                      5.0  5.7 5.9  6.4  5.0 15.2 24.3 25                               permanent strain (%)                                                          Resonant frequency                                                                        2.8  2.8 3.2  3.4  3.3 4.0  4.3  4.3                              (Hz)                                                                          6 Hz transmittance                                                                        0.30 0.30                                                                              0.45 0.50 0.45                                                                              0.70 0.68 0.67               __________________________________________________________________________

We claim:
 1. A method for producing a polyurethane flexible foam, whichcomprises reacting at least one high molecular weight polyol selectedfrom the group consisting of the following polyoxyalkylene polyol and apolymer-dispersed polyol containing the following polyoxyalkylene polyolas the matrix, an optional crosslinking agent and a polyisocyanatecompound in the presence of assisting agents selected from the groupconsisting of a catlyst, a foaming agent and a foam stabilizer or amixture thereof:Polyoxyalkylene polyol: A polyoxyalkylene polyol withthe hydroxy value (X mgKOH/g) and the total unsaturation degree (Ymeq/g) being in the following relation, with Y being not more than 0.07,the hydroxyl value (X) being from 5 to 38 and the number of hydroxylgroups being from 2 to 8:

    Y≦0.9/(X-10)


2. The method according to claim 1, wherein the polyoxyalkylene polyolhas a hydroxyl value (X) of from 5 to 28 and a total unsaturation degree(Y) of not more than 0.04.
 3. The method according to claim 1, whereinthe polyoxyalkylene polyol is a polyoxyalkylene polyol having anoxypropylene group content of at least 70% by weight and optionallycontaining oxyethylene groups.
 4. The method according to claim 3,wherein the polyoxyalkylene polyol is a polyoxyalkylene polyolcontaining at least 5% by weight of terminal oxyethylene groups.
 5. Themethod according to claim 1, wherein the polymer-dispersed polyolcontains from 1 to 40% by weight of fine polymer particles.
 6. Themethod according to claim 5, wherein the fine polymer particles are fineparticles of a vinyl polymer.
 7. The method according to claim 1,wherein the crosslinking agent is used in an amount of from 0.2 to 10parts by weight per 100 parts by weight of the polyol.
 8. The methodaccording to claim 1, wherein the crosslinking agent is a polyol havinga molecular weight of not higher than 600 per hydroxyl group and havingfrom 2 to 8 hydroxyl groups.
 9. The method according to claim 1, whereinthe polyisocyanate compound is an aromatic polyisocyanate.
 10. Themethod according to claim 9, wherein the aromatic polyisocyanate is atleast one member selected from the group consisting of tolylenediisocyanate, diphenylmethane diisocyanate, crude diphenylmethanediisocyanate and at least one modified product thereof.
 11. The methodaccording to claim 10, wherein the aromatic polyisocyanate is a mixturecomprising at least one member selected from the group consisting oftolylene isocyanate and modified products thereof and at least onemember selected from the group consisting of diphenylmethanediisocyanate, crude diphenylmethane diisocyanate and modified productsthereof.
 12. The method according to claim 1, wherein the foaming agentis water, or water and a low boiling point halogenated hydrocarbon. 13.A method for producing a polyurethane flexible foam, which comprisesreacting a polymer-dispersed polyol wherein the matrix is the followingpolyoxyalkylene polyol and fine polymer particles are dispersed stablyin the matrix, from 0.2 to 10 parts by weight, per 100 parts by weightof the polymer-dispersed polyol, of a crosslinking agent and an aromaticpolyisocyanate in an amount of from 0.8 to 1.3 equivalent per equivalentof the total amount of the polymer-dispersed polyol, the crosslinkingagent and water when a foaming agent contains water, in the presence ofa catalyst, a foaming agent and a foam stabilizer:Polyoxyalkylenepolyol: A polyoxyalkylene polyol having an oxypropylene group content ofat least 70% by weight, a terminal oxyethylene group content of at least5% by weight, a number of hydroxyl groups of from 2 to 8, a hydroxylvalue (X mgKOH/g) of from 5 to 38 and a total unsaturation degree (Ymeq/g) of not more than 0.07, with X and Y being in the followingrelation:

    Y≦0./9/(X-10).


14. The method according to claim 13, wherein the polymer-dispersedpolyol contains from 1 to 40% by weight of fine polymer particles. 15.The method according to claim 14, wherein the fine polymer particles arefine particles of a vinyl polymer.
 16. The method according to claim 13,wherein the polyoxyalkylene polyol has a hydroxyl value of from 5 to 28and a total unsaturation degree of not more than 0.04.
 17. The methodaccording to claim 13, wherein the crosslinking agent is apolyoxyalkylene polyol having a molecular weight of not higher than 300per hydroxyl group and a number of hydroxyl groups of from 3 to
 8. 18. Apolyurethane flexible foam obtained by using at least one polyolselected from a polyoxyalkylene polyol having an oxypropylene groupcontent of at least 70% by weight and a polymer-dispersed polyolcontaining said polyoxyalkylene polyol as the matrix and apolyisocyanate compound as the main reactive materials, and having aresonant frequency of not higher than 3.5 Hz.
 19. The polyurethaneflexible foam according to claim 18, wherein the impact resilience ofthe core is at least 70%.
 20. A polyurethane flexible foam obtained bythe method according to claim
 1. 21. The polyurethane flexible foamaccording to claim 20, wherein the resonant frequency is not higher than3.5 Hz.
 22. A polyurethane flexible foam obtained by the methodaccording to claim
 13. 23. The polyurethane flexible foam according toclaim 22, wherein the resonant frequency is not higher than 3.5 Hz.